Last November, the Intergovernmental Panel on Climate Change released its fourth report, a sprawling synthesis of what climate scientists know about global warming. The message was starkly grim — but to appreciate it, you had to spend some time unpacking carefully assembled passages like this:

“There is medium confidence that approximately 20-30% of species assessed so far are likely to be at increased risk of extinction if increases in global average warming exceed 1.5-2.5oC (relative to 1980-1999). As global average temperature increase exceeds about 3.5oC, model projections suggest significant extinctions (40-70% of species assessed) around the globe.”

Translation: While it’s not certain, it’s definitely in the realm of possibility that global warming will put a sizable fraction of the world’s biodiversity at risk of extinction.

If somewhere between 20 and 70 percent of species eventually do become extinct, man-made global warming will have triggered one of the largest bouts of mass extinctions in the history of the planet.

While such a huge die-off is possible, it is by no means certain. One way to narrow those certainties down is to look back at how mass extinctions unfolded in the past. “We cannot run an experiment on the Earth, so there’s a benefit to studying these ancient events,” says Michael Benton, a paleontologist at the University of Bristol in England.

As is always the case when scientists study things that happened millions of years ago, the clues about mass extinctions are scarce and their mysteries vast. But paleontologists and geologists are still managing to create a fairly detailed picture. And when you place it alongside the picture from the IPCC report, it’s a disturbing one.

The patterns of mass extinctions in the past suggest that the IPCC’s projections of extinctions from global warming are far too conservative. Shooting carbon dioxide into the atmosphere doesn’t simply harm wildlife by raising the average global temperature. It triggers a chain of events that is far more devastating than heat alone.

The most famous mass extinctions were the ones that took place 65 million years ago at the end of the Cretaceous Period. About half of all species on Earth disappeared; among the victims were most dinosaurs (with the exception of birds).

These mass extinctions became famous thanks to a startling hypothesis scientists made in the late 1970s: The extinctions were caused by the impact of a giant object from space.

Three decades later, an overwhelming amount of evidence indicates that indeed a 10-kilometer-wide asteroid dropped into the Gulf of Mexico. Many scientists argue that the effect of the impact would have been more than enough to cause the Cretaceous mass extinctions. It triggered forest fires, a dark pall that hung over the planet for months, acid rain, and other forms of environmental devastation.

Since the solar system is teeming with asteroids and comets, a number of scientists suggested that impacts might be the trigger for all the mass extinctions on Earth. What few people realize, however, is that today the Cretaceous impact looks like a rare fluke, not a general rule. “Ten years ago, I would have bet that by now some good evidence for another impact-extinction would have emerged,” says James Powell, a geologist who directs the National Physics Science Consortium and author of Night Comes to the Cretaceous. “But so far, it has not.”

Many experts now believe that other mass extinctions did not have an extraterrestrial trigger. An earthbound cause is far more likely: the sudden release of greenhouse gases, leading to rapid global warming.

Long before humans started lofting carbon dioxide into the atmosphere, Earth was creating its own greenhouse effect. Carbon dioxide and other heat-trapping gases rise from the interior of the planet through volcanoes and other outlets. They kept the planet far warmer than it would have been without an atmosphere. Over millions of years the levels of greenhouse gases gradually rose and fell, depending both on how much gas the Earth released and how much it was drawing back in as carbon settled on the ocean floor.

Geochemists reconstruct this climate history from the chemistry of rocks that formed at the time. For example, elements found in rocks, such as carbon and oxygen, come in heavy and light forms known as isotopes. The climate when a rock is forming helps determine which isotopes it will end up with. The understanding that comes from these sorts of studies is far from precise, but the margins of uncertainty are shrinking as they invent new ways to extract clues.

As the history of Earth’s climate comes more and more into focus, paleontologists are comparing it to the history of life. They’re discovering some striking parallels. Researchers at the University of York, for example, compared the rate of extinctions over the past 520 million years to changes in the planet’s temperature. They found a consistent pattern. When the planet gets warmer, the diversity of life goes down.

For much of the history of life, the rise of temperature and extinctions was slow, taking millions of years to become detectable. But the history of Earth’s climate has also been punctuated by relatively abrupt changes. And in some cases, those swings have coincided with some of the biggest, fastest bouts of mass extinctions on record.

For experts on mass extinctions, the most interesting time in the history of our planet came 250 million years ago, when the Permian Period ended and the Triassic Period began. “Everyone would agree that it’s the biggest mass extinction of all time,” says Benton. Estimates vary, but it looks as if 80 to 90 percent of all species vanished.

Entire orders were wiped out, snuffing out major lineages in an exceptionally short period of time. The ecological devastation was on a mind-boggling scale. “There are no forests or coral reefs for twenty to thirty million years,” says Benton.

Most experts now agree that these extinctions were probably triggered by a colossal belch of lava in Siberia that occurred at the end of the Permian Period. It was not the explosive volcanic eruption we’re most familiar with, but instead a steady flow of molten rock rising out of the ground for thousands of years. These flood basalts, as they’re known, eventually spread out across a region of Siberia three times as big as Texas.

The flood basalts released huge amounts of carbon dioxide into the atmosphere. As the hot lava spread out over limestone and coal deposits, it may have baked them, releasing methane, which is a particularly potent greenhouse gas. “I think that a lot of the greenhouse gases come from the baking and not the lavas themselves,” says Lee Kump, a geochemist at Pennsylvania State University.

With so much carbon dioxide and methane in the atmosphere, temperatures rose drastically, and many scientists suspect that the changing climate alone probably had a devastating effect on biodiversity. Many species just couldn’t adapt to the rapid changes. But heat was only one of several plagues visited on the Earth by the Siberian flood basalts. When carbon dioxide dissolves into sea water, it becomes acidic. In an acidic ocean coral reefs probably couldn’t build their skeletons, and clams and other animals couldn’t build their shells.

The oceans changed in other ways, according to Kump. As it grew warmer, it held less oxygen. The deep ocean suffered the biggest loss, because it was so far from the atmosphere. As the deep ocean lost its oxygen, a new series of chemical and ecological changes took place. Animals and microbes that depended on oxygen struggled to survive. And microbes that could not survive in oxygen-rich water began to thrive.

Some of those microbes, known as sulfate-reducers, are still around today in some places where oxygen is low. “Any time you scoop your hand into some mud on the shore and smell that horrible rotten egg smell, you’ve grabbed billions of sulfate-reducing bacteria,” says Kump.

That horrible smell comes from hydrogen sulfide, a nasty gas the microbes produce as they break down sulfur-bearing compounds. Sometimes know as sewer gas, it’s harmless at low levels. But as the concentration increases, sewer gas can cause headaches and interfere with the nervous system. At higher concentrations, it can kill a person in five minutes. Kump’s research indicates that as sulfate-reducers exploded in the deep ocean, they would have generated enormous amounts of hydrogen sulfide. Together, these two changes to the ocean may have driven many more species extinct.

Recent studies on fossils from 250 million years ago offer some support for this hypothesis. The first species to become extinct lived in the deep ocean, followed by relatives in shallower regions. That’s what you’d expect from an expanding pool of deadly water starved of oxygen and flush with sewer gas.

Eventually the sewer gas would have also reached the atmosphere, although Kump and other researchers have yet to pin down how much would have ended up there. At the moment, Kump doesn’t think there was enough to kill animals or plants directly. But computer models produced by scientists at the National Center for Atmospheric Research indicate that even modest levels of sewer gas would combine with methane to attack the ozone layer. Normally the ozone layer protects life below from dangerous radiation from space. If the sewer gas and methane were potent enough to destroy much of the ozone, the effects could have been devastating.

There’s some evidence to suggest that such devastating effects did in fact take place. For example, some scientists have observed that many fern spore fossils from 250 million years took on freakish shapes. They argue that radiation coming through a weakened ozone layer could have triggered widespread mutations.

The new IPCC report does not delve into any of this research. The projected rates of extinctions they offer are based mostly on studies of a single effect of global warming: how an increase in temperatures will shift the ranges of species. In some cases, computer models suggest, the ranges will expand. In others they will shrink. Species forced into smaller ranges may shrink in population, and thus become more vulnerable to inbreeding, diseases, and various catastrophes. Any extinctions caused by low oxygen in the ocean or a disappearing ozone layer would come on top of extinctions from shifting ranges.

Kump and other experts on past mass extinctions are not ready to start making projections of their own. But they do have a sober piece of news about the current bout of global warming. We are injecting greenhouse gases at a rate, says Kump, “that rivals or exceed anything we’ve seen in the geological record.” If the geological record tells us anything, it’s that such sudden rises of greenhouse gases can trigger huge effects.

But don’t expect to hear a forecast today for clouds of sewer gas to come in off the ocean in the morning. “It’s not something that would happen overnight,” says Kump. It would take thousands of years at the least for significant amounts of hydrogen sulfide to build up. It’s also possible that the chemistry of the ocean today is different in some critical ways compared to 250 million years ago, and will therefore keep sulfate-reducing bacteria in check. “It would be facile to say, ‘This is what’s going to happen,’” says Benton.

Nevertheless, the parallels between then and now are very unsettling for experts on mass extinction. “It’s too horrible to think about,” says Peter Ward, a paleontologist at the University of Washington. It’s also difficult to imagine, because it would not come in our lifetime, or even the lifetime of our great-grandchildren. But that hardly means we can ignore it.

Carl Zimmer writes about science for The New York Times and a number of magazines. He is currently a lecturer at Yale University. A 2007 winner of the National Academies of Science Communication Award, Zimmer is the author of several books, including Microcosm: E. coli and the New Science of Life and The Tangled Bank: An Introduction to Evolution. He also writes an award-winning blog about science called The Loom. More about Carl Zimmer →